1. Field of the Invention
The present invention relates to a method of preparing metal nanoparticles. More particularly, the present invention relates to a simple process of preparing metal nanoparticles on a large scale and that can be used as a fuel cell catalyst.
2. Description of the Related Art
A fuel cell is a power generation system for producing electrical energy through an electrochemical redox reaction of an oxidant and hydrogen in a hydrocarbon-based material such as methanol, ethanol, and natural gas. Such a fuel cell is a clean energy source that can replace fossil fuels. The fuel cell includes a stack composed of unit cells, and produces various ranges of power output. Since a fuel cell has a four to ten times higher energy density than a small lithium battery, the fuel cell has been highlighted as a small portable power source.
Representative exemplary fuel cells include a polymer electrolyte membrane fuel cell (PEMFC) and a direct oxidation fuel cell (DOFC). The direct oxidation fuel cell includes a direct methanol fuel cell that uses methanol as a fuel. The polymer electrolyte fuel cell has an advantage of high energy density, but it also has problems in that the hydrogen gas must be carefully handled and that accessory facilities, such as a fuel reforming processor for reforming methane or methanol, natural gas, and the like, are needed in order to produce hydrogen as the fuel gas.
On the contrary, a direct oxidation fuel cell has a lower energy density than that of the polymer electrolyte fuel cell, but it has advantages of easy handling of a fuel and is capable of being operated at room temperature due to its low operation temperature, and there is no need for additional fuel reforming processors.
In the above fuel cells, the stack that generates electricity substantially includes several to scores of unit cells stacked in multi-layers, and each unit cell is formed of a membrane-electrode assembly (MEA) and a separator (also referred to as a bipolar plate). The membrane-electrode assembly has an anode and a cathode disposed on each other with an electrolyte membrane arranged between them.
In the above fuel cells, a fuel is supplied to the anode and is adsorbed on catalysts of the anode, and the fuel is oxidized to produce protons and electrons. The electrons are transferred into the cathode via an external circuit, and the protons are also transferred into the cathode through the polymer electrolyte membrane. In addition, an oxidant is supplied to the cathode, and then the oxidant, protons, and electrons are reacted on catalysts of the cathode to produce electricity along with water.
The anode catalyst or the cathode catalyst can include various metal-based catalysts including a platinum-based metal. Recently, the metal-based catalysts have been known to include a nanoparticle shape to improve activity. In addition, the metal nanoparticles can be applied to various fields such as catalysts for fuel cell, solar cell or purification, functional particles such as nano silver, or nano gold, conductive ink for manufacturing electrodes of display, functional additives for adhesives, and the like.
Conventional methods of manufacturing metal nanoparticles include a chemical method and an irradiation method. The chemical method is the most popular, in which a metal precursor is reduced by using a reducing agent such as NaBH4, hydrazine, ethylene glycol, H2SO3, LiAlH4, and the like. However, it is not good for mass production, because it needs to be optimized by several factors such as temperature, pH, reaction speed (time) of a metal precursor, and a reactant.
In addition, the irradiation method is performed by using light rather than a reducing agent, in general, gamma ray, UV, and the like. In general, the gamma ray is the most popular among these, but it has a limit such as formation of an alloy or deterioration of reactivity when manufacturing metal nanoparticles.